There is a small probability that a double-enveloped tungsten halogen incandescent lamp will burst during operation of the lamp, hereinafter called a "containment failure" of the lamp. When a lamp containment failure occurs, the sequence of events internal to the lamp is as follows. The tungsten halogen capsule bursts causing fragments of glass or shards to be propelled against the outer envelope; these shards shatter the outer envelope of the lamp. The external result is that the lamp bursts. It is this type of lamp failure that is the subject of this disclosure.
One type of containment failure which may occur in a lamp having a single-ended light-source capsule is as follows. The capsule ruptures near or in the press seal. The remainder or upper body of the capsule remains intact. Because of the release of substantial pressure from within the capsule, the capsule body (still referred to herein as a shard) is propelled away from the press seal toward the outer envelope. The outer envelope is shattered by the impact of the propelled capsule body.
The causes of these infrequent lamp failures are varied and unpredictable. There is no known way to eliminate the possibility of such failures. Although occurrence of the failure is rare, nevertheless it could present a safety hazard to a person in the immediate vicinity of a lamp. Where such failures can be anticipated, lamp manufacturers notify users by means of warnings on packages and other descriptive materials and by suggested precautions in specifications. This hazard may be avoided by operating the lamp in a fixture designed to contain such a failure. The requirement that the lamp be operated in a protective fixture is frequently employed in commercial usage. However, this procedural safeguard is less acceptable for consumer usage. For reasons of safety, economy, and convenience in both commercial and consumer usage, it may be desirable to incorporate a reliable containment device as part of the lamp itself.
As used herein, the terms "light-source capsule" or "capsule" denote a tungsten halogen incandescent light-generating capsule of a double-enveloped tungsten halogen lamp. This type of lamp, with single-ended and double-ended capsules, has been suggested in the prior art. U.S. Pat. Nos. 3,194,625, by Danko, issued July 13, 1965; 3,448,321, by Shanks, issued June 3, 1969; and 3,515,930, by Walsh et al, issued June 2, 1970, provide examples of tungsten halogen lamps of both the single-ended and double-ended capsule varieties.
The terms "efficacy" or "luminous efficacy" used herein are a measure, expressed in lumens per watt, of the total luminous flux emitted by a light source over all wavelengths divided by the power input of the source.
For a general reference on the tungsten halogen incandescent lamp, see IES Lighting Handbook, 1981 Reference Volume, Section 8.
In U.S. Pat. No. 4,281,274, issued July 28, 1981, by Bechard et al, there is disclosed an enclosure of glass surrounding the arc tube within the outer envelope of an arc discharge lamp. The enclosure is suggested as being useful as a means to protect against a containment failure of the lamp. While such enclosure may be effective in containing some arc tube bursts, it has been found that in a substantial percentage of cases the enclosure itself is shattered by the burst and containment failure of the lamp follows. Thus, the glass enclosure device taught in the Bechard et al patent offers only limited protection against lamp-containment failures, and such protection is especially tenuous in lamps having light-source capsules in which operating pressures may be as high as 20 atmospheres.